The invention relates to a process for the addition of additives to thermoplastics. In particular, the invention is directed to adding additives to thermoplastics which in processing are obtained as a solution or melt.
Generally, in the processing of thermoplastics, e.g. polyolefins such as homopolymers and copolymers of ethylene, propylene, butene-1,4-methyl pentene-1, and the like, additives are added to said thermoplastics to improve the properties of those thermoplastics. For instance, by additives is meant, heat-stabilizers, antioxidants, UV-stabilizers, internal or external lubricants, antistatic agents, antiblocking agents, corrosion-inhibitors, and the like.
Thermoplastic polyolefins may be prepared and obtained as powders, which may be processed into granules. Additives for improving the properties of the resulting thermoplastic may be mixed directly with the powdery plastic material or the additive may be mixed with a part of the powdery plastic material to form a master batch; the master batch is added to and mixed with the remainder of the thermoplastic powdery plastics material. Thermoplastic polyolefins may also be obtained as products in solution. For instance, methods of preparing polyolefins which are obtained in solution have been developed, particularly for high density polyethylene, i.e. polyethylene with a density of at least 0.94. Such processes have been described, for example, in U.S. Pat. No. 2,862,917 and U.S. Pat. No. 3,491,073, and in British Patent Specifications 1,235,062 and 1,251,177. When solution polymerization techniques are employed to produce polyolefins, particularly polyethylene, a polymer melt is recovered which is subsequently extruded into, for instance, granules.
As set forth above, there are instances in which the addition of additives to the polyolefins is desirable. Such additives include heat stabilizers, antioxidants, UV-stabilizers, internal or external lubricants, antistatic agents, antiblocking agents, corrosion-inhibitors, and the like. The addition of measured amounts of such additives to a polymer melt presents practical difficulties.
Most additives can be dissolved. However, some additives, such as carbon black, metal carbonate and oxides, are insoluble. In overcoming such a problem, it could be possible to dissolve the soluble additives and to disperse insoluble additives, if any, in these solutions. Solutions or suspensions of this type could be fed to the extruding device in which the polymer is processed into granules.
Alternatively, in instances where one solvent may not act to dissolve all additives, it is possible to dissolve those additives in a number of solvents, prior to introducing the additives to the extruder. However, use of solvents containing such additives, which are subsequently fed to an extruder, presents certain disadvantages. In order to extrude solvent-containing material, specially adapted vented extruders must be used. Vented extruders are very expensive. The use of suspensions of additives has similar drawbacks because of the nature of the suspending agents.
The majority of the solid additives, used in the processing of thermoplastic polyolefins, can be melted. The melting points of such additives usually are less than 250.degree. C. Such fusible additives may be added in measured amounts in the molten state, but addition of measured amounts of molten additives appears to present several problems. The addition is effected gradually and it will be necessary to keep the additives in the molten state for some time. Although the aggregate of additives may contain inorganic compounds, the aggregate consists to a considerable extent of organic or metal-organic compounds which, in most cases, readily discolor at elevated temperatures. The temperatures at which the fusible components will completely melt vary widely. As the melting point increases, the undesirable discolorations increase.
To limit discolorations as much as possible, the temperature of the melted additive must be kept as low as possible, i.e. at or slightly over the melting point of said additive. Even then, undesirable discolorations are often found to occur. If the temperature drops below the melting point, the molten mass sets substantially or completely to a hard solid cake. Local cooling can readily occur in feed pipes and valves. The formation of a hard solid cake will easily give rise to plugging.
The invention aims at providing a process for the addition to thermoplastic of additives that, at least for the greater part, can be melted at temperatures below 250.degree. C.